For the mousse:
Melt chocolate in a double boiler.
Beat egg yolks with confectioners sugar until light an thick. Beat in the melted chocolate until mixture is smooth.
Gradually add the soft butter and sifted cocoa, beating until mixture is well blended. Whip the cream to a medium peak.
Fold in the chocolate mixture.
Spoon mousse in a decorative mold. Chill for 2 to 3 hours.
For the orange sauce:
Boil the orange juice and reduce to half. Beat the eggs with the sugar. Add the scalded milk and orange rind. Cook over low heat, stirring with a wooden spoon until thick. Do not boil. Add orange juice. Refrigerate. Unmold mousse. Serve with the sauce.